Creep testing of high-temperature alloys



Dec. 1, 1953 T. C. VAN DEGRIFT CREEP TESTING OF' HIGH-TEMPERATURE ALI..

OYS

Filed Oct.

2 Sheets-Sheet l En fa/maf@ @WQ/v5.77?"

(itornegs Dec. l, 1953 T. c. VAN DEGRIFT 2,660,881

CREEP TESTING OF HIGH-TEMPERATURE ALLOYS Filed OCT.. 29, 1947 2 Sheets-Sheet 2 nventor /a I Gttornegs Patented Dec. 1, 1953 UNITED STATES OFFICE.

CREEP TESTING OF HIGH-TEMPERATURE ALLYS Application October 29, 1947, Serial No. 782,855

(Cl. 'i3-15.6)

4 Claims.

The present invention relates to creep testing equipment for high temperature alloys such as used in gas turbine blades.

More particularly the present invention relates to a method of, and apparatus for subjecting test specimens to tensile and vibratory stresses while heating said specimens to a high temperature, and measuring the eiTect of said stresses and heating.

The ability of gas turbine blading materials to withstand high temperatures and loads is one of the principal limiting factors in gas turbine design. The development of a blading inaterial which could withstand higher temperatures would result in much higher gas turbine eiiiciency.

In one of the modern jet propulsion engines the blades in service are subjected to the combined effects of the following:

(a) The turbine blades operate at a temperatrue of from 120D tol 1500 degrees Fahrenheit.

(b) Centrifugal tensile stresses in the blades reach a magnitude of 25,000 pounds per square inch.

(c) vibra-tory stresses are set up Within the blade itself at various high frequencies.

(d) rlhe action of hot combustion gases tends to disturb the physical characteristics and surface condition of the blades.

Considerable data on the properties of alloys at high temperatures has been collected by many investigators using equipment available before the present invention. There are two principal types of laboratory tests which have previously been used for determining the suitability of alloys for gas turbine blades. One of these tests consists of subjecting the alloys in question to high ternperature and static tensile load. When tested under these conditions the material may be deformed in a plastic manner or creep. This gives static tensile characteristics of the test specimen under high temperature conditions. This testl may be either carried outas a creep test oras la creep to rupture test.

small constant load for several thousand hours and is commonly used in testing gas turbine blade material, The second principal test used toA test gas turbine blade material is a reverse bending test. This test consists of subjecting the material to bending stresses in alternate directions while maintained at a high temperature.

These two tests have been commonly used since it is very desirable to obtain characteristics of The creep test consists of subjecting the test specimen to a relatively gas turbine blade alloys under both bending and tensile strains.

The aggregate results of these.

` is reliable to operate lest specimen 2 is mounted in the having anupper housing 4 and a base member is' v It is a further object of the present invention to produce a machine for subjecting test specimens to static tensile loads, vibratory transverse loads and hot combustion gases simultaneously, this machine to be cap-able of accurately applying these three quantities and measuring the specimen deformation resulting therefrom.

It is a still further object of the present invention vto produce a test machine that will reproduce service conditions of gas turbine blades and Other objects of Figure 1 with parts broken away and shown in section.

Figure 4 is a partial sectional view of the gear box containing part of the follow-up mechanism.l

Figure 5 is a schematic showing of the followup mechanism designed to maintain constant` load on the test specimen.

Referring more particularly to Figure l, the

connected by tubular pillars 8 and lil.

The test specimen while thus supported is sub- 7 jected to the combined effect, of hot combustion gases, static tensile stresses, and transverse vibratory stresses. The adjustable' gas burners Il, !2,..=|3, le, l5, I6, Il and I8 cause hot gases to impinge on the heat shield lll specimen. This shield is open at both ends thus exposing the test specimen to lthe hot gases. It does, however, prevent the burner flame from y impinging directly on the test specimen and causing hot spots with resultinginaccurate-test-- results. rI'he control probe 2o of conventional and simple in construction. f the' present invention Will become obvious upon reading the specification and, inspection of the iigures of the drawings and test' machine to heat the test construction is used to turn off the gas supply in the event the burners are for some reason accidentally extinguished. The test specimen 2 is supported between clamps 22 and 24 so as to receive the static tensile and transverse vibratory stresses. The tensile: leading system includesL motor 26belt: drive 28, gear box il, .sprocket 32",. chain 36, sprocket 36, spline shaft 38, gear 40, threaded cap 42, loading coil spring M, together. with other necessary elements. A follow-up system for maintaining the tensile stress system equilibrium and for recording the amount of' strain is included in the housing 4f, also includes follow-up drivemntor Anand Selsym generator t3 located on the housing. The transv verse vibratory stress system includes a syn.- chronous motor 50 of desired site andspee'd,y together with a drive shaft 52 and unbalanced Weights 5t. The motor 50 is directly connected tothese unbalanced Weights 54- andY by means. of. these Weights. cause unbalanced stresses which; resulty in transverse strain. oflthe4 test., specimen. Arr oilV circulatingsystem. including. an. inlet |38 and an outlet IESA is provided for lubricatingthe. bearings 56 and 5& and supplying oil tothe coolingjet..

Referring to. Figure 2;. the. gas,v burners: ll through. i8'. are shown in` greater' detaiL. together with the thermocouples d'3 usedl to control the burners and to record the temperature of the test specimen.v As4 showninthis. figurev the test specimen is attached tothe upper member 22 by means of collars B' and' 547... It will be noted thermember 2.2 has two mating halves whichper mit the clamping of the test' specimen 2 so as to.Y hold it rigidin. a transverse. direction and` exert. longitudinal tensile stresses. The clamping member 22' is secured to a member BE in such a, way asv to be adjustably supportedby screws BtA and. lll and be maintained in. fixed alignment. by means of projection l2. in thel member 'Ml mating depression '[5 in the member lill.. Thel ad.- justing screws 6d and; 'l'iipermit4 movement-of.' the.y member Se relative to member M so asto iacilitate the coaxial alignment. of the member Z1 with the member 2:3- The member lll-is secured. to. the tensile loading bar lll and'lto metal diaphragm dll.

Referring to the lower part; of Figurer 2', the clamping member 21lis identical tothe member 22 mentioned above.A tially the same as member li which is held by the member Z2 above. TheV membersb' and. $2.1 in addition to acting as supporting and stress. transmitting members, also operate as heat dams to minimize the transfer of heat' from the test specimen heating soneto the membersd and 01 The member 82 is' attached' to the housing 'il by meansof mating projection 354' and depression 8E'.` This permits the introduction of the vibratory forces; set up. by the rotatingvzeights. 54' through the housing iin-to the testspecimenl.. Theft/fibratory system includingr the Weights- 54 supported. in the housing byvv means` of. bearings; 5.a and 58 and is driven. by means' ofA The tubular shaft 52 has a second'.purpose;A This; second purpose. istov transmi Vlubricating ami'` cooling fluid. to` the4 bearings et? and 58I and. also. to the cooling jetA 88. rllhe oi'l from this cooling jet 83 impinges on the member 9U cooling this; member and also the oher portions ofthe nous# ing 60. The oil then, under thefforce-of gravity, proceeds through the openings 92 and Q5" to= the cavity between the tubular rotating member 5e andthe tensile stress n'ieiriberV Mback to the. oil? The member. Si is substanstoppingA the motor 36.

4 outlet |39. The oil pump for circulating this oil may be of any suitable type.

The gear 40 is rigidly attached to thefthreaded member 42. Figure 3 is an extension of Figure 2 and referring to Figure 3 it will be noted that this screw,- Almatestwith a member da. Thismember llisrrigi'dlyf attached to theepillars 8. andi L0 mentioned in connection with Figure 1. The coil spring 44, through plate 98, exerts a tensile stress on the tubular member 94. The plate e8 is maintained in. apxed position radially but is allowed atl-imited freedom of axial movement by means ol diaphragm. Luth This system carries thereon movable; contacts [|32 and |643 which mate with t stationrer contact. LBS to energize the follow-up system., which will be mentioned later.

Referring to ligme 4, the upper portion o the housing@ isshown in sectional form. In this ngure it Will be noted that the member 1S has a threaded, portion |08- This threaded portion mates with an. internally threaded portion of the gear member. H0. The gear |50 isl driven by spur gear |2 which is, rigidlyv attached to larger spurv gear. IU?.` byy reason of their both being keyed to the. samer shaft. Larger spur gear H4 is driven by' smaller' spur gear H6, which gear is in turn rigidly' attached to gear ||A since they are keyedto a' commonl shaft. The gear H8 is driven by spur, gear |20 which spur gear is rigidly attachedv to gear |22. The gear |22 is` connected tothe gear box 'll, see Figure 1 by means of a spur gear' |281 This geark system ispart oi the followup system necessary to maintain the system inl equilibrium. as' Willbe described later. In order to determine and record the amount o more ment in an axial direction of the clamp 'r2-2V and hence' determine the tensile deformation ci thetest piece, a, Selsyn generator 4S is connected to the gear I ||l by'meansofa small spur gear |28.

Referring to Figure' 5, this follow-up system is shown inv a schematic' form. The tensile system is1 soAV designed that all measurable changes in elongation occur in the necked down portion ofthespecimen 2. Itr willftherefore be seen that as the force` exerted through the coil spring mi' causes deformation of4 the test specimen, the movable contactr H52 Will approach a stationary contact |06. When an electrical circuit is completed' through these contacts the electronic re layV |30 causes the motor a6 to rotate in such direction asY to move theA upper securing member' 7E' up in avertical direction. This movement is re'ected through the rest of' the system causingthe Contact H32 to break with contact |68 thus With this sysem thus operating the diaphragm |00- is maintained in a horizontal position so that all deformation of the .test specimen 2- is reected in vertical movement of the member 13; This vertical movement'is recorded by means of theY Selsyn generator 48* and' a matingSelsyn receiver vZilli.

Having thus described the apparatus in general, iti is now considered desirable to discuss the systems included therein individually.

Static. load. adjusting system and capable of having a load of 4500 pounds was found to be suitable in one particular application of the present invention This spring beof known rate, it: is possible to determine the load which it exerts measuring the amountY it is: compressed. The. bottom end ofthis spring is maintained in a fixed position by means of the follow-up mechanism described in connection with Figure 5 above. The spring is compressed from its upper end by means of a loading system driven by motor 25. This motor, through belt drive 26 and gear box 3), drives a chain sprocket i327. Motion is transmitted from this sprocket to the driving gear` or splined shaft 3B by means of a chain 34 and a mating chain sprocket 3b located on this gear or shaft. This shaft causes the member 132 to be rotated and thus screwed into the member 96 by rotating gear 60. The amount this gear di! is rotated and thus the amount the spring is compressed is measured by the rotation counter 432. This counter may be calibrated to read in pounds force for a given spring or in pounds per square inch exerted on the test specimen for a given rate loading spring i4 and a given diameter specimen 2. As the load is exerted on the test specimen 2 it deforms in a longitudinal direction causing the contact E02 to approach a contact F08. As a circuit is made between these contacts the motor is energized as described in Figure 4 and the plate 98 is moved upwardly until it occupies its original position. When in this original position the calibration of the coil spring sie is accurate since the member 00 does not then exert any appreciable axial force. As this follow-up mechanism driven by motor i5 is activated, the Selsyn generator d8 is moved so as to indicate the amount of deformation of the test specimen 2. The instant invention is, of course, applicable for use in testing various materials. In one particular case this invention is used to test the properties of gas turbine blade materials. In this case the tensile stresses are maintained at a value of 25,000 pounds per square inch and longitudinal `deformation over a period of time of about 400 hours is measured.

Heating system In one particular embodiment of the present invention the specimen is heated by means of eight gas burners. These burners are controlled and the temperature of the specimen is indicated by a series of thermocouples 63 attached to various positions on the specimen. These thermocouple wires are threaded through porcelain insulators litri and spot welded to the surface of the specimen to form a hot junction. To prevent breakage of these thermocouple wires by the vibratory stresses set up by the test machine the porcelain insulators are strapped to the specimen along the length of the thermocouple. The thermocouple wires leave the specimen at its upper end where the vibratory motion is minimum and are connected by lead wires to the heat indicating and control system.

In one particular embodiment of the present invention, one of the thermocouple wires is attached to the lower llet of the test specimen where the vibratory stresses are greatest. This thermocouple is not connected to the gas burner control system since the vibratory stresses make its usefulness quite short lived. The temperature of the test specimen under the action of the hot gases is maintained to Within the small limits of plus or minus 5 degrees. These limits are located at various points within the operating temperature of the machine for which the material will be ultimately used. In the instant case, where the material is designed for use in gas turbine blades, this temperature range is located Vibration excitation system It was found desirable to excite the test specimen with vibratory stresses which would produce a maximum strain in opposite phase at the two ends of the specimen with a minimum strain near its midpoint. After experimentation it was found that a system with an elastic line as shown by |36 in Figure 5, was Optimum. After a study of the frequency of vibratory stresses in gas turbine blades it was decided that one particular embodiment of the present invention should include a means for exerting approximately 10,000 stress cycles per minute. In order to exert the proper vibratory stresses, rotating weights of proper magnitude of unbalance were located in a housing 6U below the test specimen. The eX- citer motor in the three phase induction motor is supplied with cycle current. The force excited by the rotating out of balance weights cause the bottom of the test specimen to move in a circle, with the centerline of the lower sus pension system, that is, the lower chuck alignment head exciter case, and tube, generating the surface of a. cone whose apex is at the center of the bottom diaphragm. The upper alignment head is held laterally rigid as previously mentioned, This system gives an elastic line as shown in |35 with a vibratory stress distribution which is maximum at the two ends of the specimen. In one particular embodiment the lower end of the specimen has a stress measured by means of a conventional wire strain gage of plus 13,000 pounds per square inch at the same instant the upper end of the test specimen has a stress of minus 9,000 pounds per square inch. This stress is obtained with exciter weights having a total imbalance of 2.4 oz. inches.

Cooling and lubrication of exciter case 'Ihe exciter case being located in the of the burners is subjected to an appreciable amount of heating. In order to properly lubricate the bearings carrying the unbalance weights it is necessary that a large volume of oil be introduced to the case. In the present invention an oil pump, not shown, supplies oil to the inlet pipe 13S and exhausts it from the pipe 39. oil is pumped through the opening in tubular bearings also pumped through jet 88 to cool the case. The oil returns to the pump through the opening between the tubular shaft 52 and the tubular shaft 9 and through the outlet |39.

Elcngatz'on recording the specimen and the suspension'system. As mentioned above, the cross sectional area of the suspension system is considerably greater than that of the test specimen and also the suspension system is not subjected to the high temperature bearing on the test specimen. As a result practically all ci the plastic deformation ofv the system occurs in the gage length of the test specimen. It may therefore be seen that the movement o the load adjusting screw relative to the frame o the machine is a measure of specimen elongation. This elongation is measured and recorded by means of the Selsyn system including the transmitter 8 and receiver 20e shown in Figure l. 1

' Test procedure After the test specimens have been prepared the thermocouples are attached and the specimen placed in the testing machine. Only a few minutes is required to bring the test specimen up to temperature. It is necessary to leave the `ourner on for a period of twenty hours before all thermal deformation o the machine takes place. Ater this period of stabilization or" the temperature, the static tensile and transverse vibratory stresses are introduced. The operation of the machine is continued from this period on to the end of the test period or to the rupture of the test specimen.

lt is to be understood also that although the invention has been described with specic reference to a particular embodiment thereof, it is not to be so limited, since changes and altera- 'ions therein be made which are within the 4full intended scope ci this invention as denned by the appended claims.

I claim:

l. Apparatus for determining the effect o tensile and vibratory stresses on a material at a high temperature including: a pair of attaching members adapted to support a test piece, resilient means attached to one oi said attaching members, means to apply a force to said resilient means to create a tensile stress in the test piece, electrical means responsive to movement of said one attaching member operable to move the other of attaching members so that the tensile stress will be constant regardless of elongation strain of the test piece, means to apply a vibrating stress to the test piece and including rotating eccentric Weights acting through a jour-- nal to transmit vibratory forces to the said one attachingr member and including means to limit vibration of the other of said attaching members, means to apply heat to the test piece including a. shield surrounding said test to apply heat to the exterior of said shield to maintain the test'` piece at a constant temperature, means to measure the elongation stress oi the test piece, and means permitting the determination of the tensile force applied to the test piece.

2. In apparatus for determining the physical characteristics oi a rigid material including holding members to hold a test bar in the apparatus, a static loading system arranged to exert a tensile stress on the test bar of said material, vibratory means consisting of unbalanced weights adapted to be rotated in an axis parallel to the direction of the application ci the tensile stresses for simultaneously exerting transverse vibratory stresses into said test bar, means to transmit vibrations due to rotation of the Weights to one of the holding members, and a heating` system for heating said test bar while exerting said tensile and vibratory stresses.

3. Apparatus for determining the physical characteristics of a test specimen including: tWo attaching members for supporting a test piece, stress means to resiliently apply tensile force to the rst of said attaching members, moving means operable to longitudinally move the second of said attaching members, control means to govern said moving means to move the second attaching member in accordance with any tendency of the first attaching member to move longitudinally so that the tensile force remains constant upon strain elongation of the test piece, and vibratory means for applying a vibratory transverse stress to said test piece, said vibratory means includingmeans to transversely vibrate said first attaching member and means to hold said second attaching means against said transverse movement.

fi. Apparatus for determining physical characteristics as claimed in claim 3 wherein said vibratory means includes eccentrically rotatable members of known Weight acting through bearings on said first attaching means.

THOMAS C. VAN DEGRIFT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 22,415 Eksergian Jan. 1l, 194e 1,193,686 Heisler Aug. 8, 1916 1,652,525 Hahneman etal. Dec. 13, 1927 1,906,340 Scott May 2, 1933 2,37 5,034 Semchyshen May l, 1945 2,436,317 Manjoine Feb. 17, 1948 piece, and means' 

